The sprague clutch is commonly employed as a one-directional clutch, for example in vehicle transmissions. FIG. 18 shows a prior art type sprague clutch. A tubular body 100 has a plurality of axial recesses 101 formed therein to define a plurality of circumferential ramp surfaces 102. Rollers 103 are located within recesses 101 and maintained in a biased position by springs 104. Rollers 103 contact and engage a ground inner ring 105.
When inner ring 105 is rotated clockwise with respect to body 100 rollers 103 are urged by inner ring 105 in a clockwise direction. Due to inclined ramp surfaces 102 rollers 103 become wedged between ramp surfaces 102 and inner ring 105. Clockwise rotational force is therefore transferred between outer race 100 and inner ring 105 through rollers 103. When inner ring 105 is rotated anticlockwise with respect to outer race 100 rollers 103 move anticlockwise with respect to outer race 100. This frees the rollers and inner ring 105 is able to rotate relatively freely with respect to outer race 100.
A modified design is shown in U.S. Pat. No. 5,103,950 in which roller recesses having a double cam-profile are utilized. Rollers 30 are also located within a cage 40. The double earn profile 21 enables the clutch to work in either direction. When shaft 10 and outer race 20 are wedged together by rollers 30, cage 40 is driven via shaft 10 and rollers 30. Power is thus transferred via shaft 10, rollers 30 and cage 40. This clutch arrangement works automatically so that relative rotation of shaft 10 in outer race 20 causes rollers 30 to lock them together and cage 40 is driven thereby.
During tubular running operations or well operations it is necessary to grip an oil field tubular, such as a drill string, to hold one section whilst another section is rotated relative thereto to assemble or disassemble a string. A number of automated devices for handling drill pipes are known such as disclosed in U.S. Pat. No. 4,585,079.
Standard oil field tubulars are relatively robust and prior art clamping mechanisms have included toothed jaws or other means which apply a high force to a relatively small area of the tubular. As standard drill string is formed of relatively uniform steel this does not pose a major problem. However, some modern drill strings are formed of special materials, such as high alloy steel pipe (e.g. 13 Cr), and handling by conventional clamping mechanisms can damage them. Such damage may reduce the life of the drill string and reduce the advantage gained from any surface coating or treatment.
For example, conventional methods of gripping a downhole tubular are described in U.S. Pat. No. 5,845,549 and U.S. Pat. No. 4,084,453. Gripping members with sharp teeth are forced into engagement with the tubular. A problem with this method is that the teeth cause permanent deformation of the tubular. In certain circumstances this can present serious problems. For instance, in a sour gas well, corrosive gases such as Hydrogen Sulphide and Carbon Dioxide will be present. If the tubular is formed with a material such as Chromium, these gases will corrode the tubular more quickly if the tubular has a rough deformed surface.
The situation is problematic as the clamping mechanisms must restrain oil field tubulars against high torques (up to 300,000 foot pounds) in a dirty and greasy environment. Further, the tubulars may vary in diameter, ovality, wall thickness, material etc. The tubulars may also be dented, rusty or have surface scale.
A number of tools include ratchet mechanisms to allow free rotation in only one selected direction. Such tools include wrenches for socket sets, screw drivers, braces etc. In some situations, especially with wrenches, there is very limited space to move a tool. In some cases the ratchet steps may be so coarse that it is not possible to move between one ratchet position to the next. This may make it impossible to use a tool in a confined space.
Pipe clamps typically include jaws which grip a pipe to be retained. As with oil field tubulars this may result in marking of the pipe. It would be desirable to provide a pipe clamp which is able to resist high rotational forces without damaging the pipe.